The purpose of this collaborative multicenter study is to better understand the course of HIV in women, the determinants of transmission of HIV from women to their infants, the course of HIV in infants and the effect of pregnancy on the course of HIV in women. This is a prospective, natural history study of HIV infected pregnant women and their infants. They are followed at fixed intervals, more frequently during pregnancy and infancy, and less frequently after the postnatal visit and the first year of life, respectively. The schedule of visits is consistent with what would be standard of care if they were not on a protocol. History, physical examination, and blood and urine tests are repeated at one to six month intervals till three years post partum for the women, indefinitely for the infected infants, and for at least two years for the uninfected infants.